Many scientific studies and procedures in various fields (e.g. field of medicine) require the detection or sensing of target molecules. For example, early detection of certain diseases requires detection of biomarkers that are present in low level of concentrations during the early stages of the disease. Nucleic acid diagnostics use blotting techniques, polymerase chain reaction and fluorescence methods to undertake the detection of target molecule. However, these techniques usually require samples with high concentration and fluorescence labelling. Labelling has been useful in some cases, but can be challenging in other cases. Although inexpensive, labelling-based sensing methods are generally prone to provide false positives due to the so-called cross-reactivity or interference with other components in the sample under test leading to inaccurate results. Moreover, the surface plasmon resonance (SPR) sensing technique has gained prominence in the sensor world due to its high sensitivity. However, SPR sensor systems in general have a small detection area, which makes the sensing process less efficient and the detection of molecules in low concentration more challenging.
Further, optical sensing approaches using optical fiber offer a low-cost solution for molecule detection. For example, Nan Zhang, et al., published a technical paper entitled “Side-channel photonic crystal fiber for surface enhanced Raman scattering sensing”, that discloses an optical sensor that uses side-channel photonic crystal fiber (SC-PCF) for surface enhanced Raman scattering (SERS) sensing platform (see FIG. 2). The SC-PCF has solid core with air holes cladding arranged in triangular lattice structure, wherein one-third of the said cladding is intentionally removed to facilitate the flow of liquid and the interaction between the liquid sample and the fiber core guided lightwave. The said sensing system requires an objective lens to deliver laser into the solid core of the SC-PCF. This objective lens requires careful alignment with the SC-PCF, making this design of sensing system not fit for applications that require portable sensor and fast analysis. In addition, the said sensing system also uses free space coupling as its approach to couple light into the liquid sample. Free space coupling is quite lossy as evidenced by the result of their experiment wherein transmission loss is estimated to be around −45 dB for fiber length of 1 meter and shorter. This means that the said sensing system will require high power laser system to ensure high sensitivity sensing. Given these limitations associated with current approaches for molecule sensing, there is a need for alternative molecule sensing solution that will address those limitations. Herein, sensing solutions are provided that are low-cost, portable, efficient, and sensitive enough to detect a single molecule and molecules in wide range of concentration (e.g., low concentration to high concentration).